The conventional gas turbine fuel control is complex electro-mechanical device that uses a number of engine operating conditions (parameters) to regulate fuel flow to the burner to achieve and maintain a commanded engine speed, such as fan speed N1. The fuel control, using feedback, responds to power lever setting (PLA) to match commanded power and fan speed. Among the engine operating parameters that the control typically uses are N1 and N2, respectively the speed of the low and high speed rotors. Other parameters include the temperature and pressure at the inlet and within the compressor stage and exhaust nozzle orientation, in the case of high performance engines employing variable pitch and area exhaust nozzles.
Depending on engine and flight conditions, such a command for peak acceleration from cruise, the control may select one parameter over another on which to "close the loop" for fuel flow to the engine. The transfer function for the control path for each parameter is a so-called proportional integral control, which provides good response and accuracy for aircraft engine applications. The basic transfer for fuel flow WF may be expressed as: EQU WF.sub.t =K1.multidot..intg.WF.sub.Return +K2.multidot..DELTA..delta.t (1)
where WFt is the total fuel flow at time t and error is the parameter feedback, such as the value of N1 (closing the loop on N1). Ideally, the output from each loop (for each engine operating parameter) produces the same scheduled fuel flow (WFR) at all times, and if that were true, selecting one loop over another would be invisible in the sense that there would be no immediate change in WFR at selection. This is not the case, however, because the parameters have different relationships to engine operation at any instant and thus one may command more or less WFR than another at any instant in time, creating a significant stability problem when selecting one channel over another. When selection is carried in this way, the loops can have significant divergence, producing erratic control.